Supporting Information. Toward Highly Reversible Magnesium-Sulfur Batteries with Efficient and Practical Mg[B(hfip) 4 ] 2 Electrolyte

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1 Supporting Information Toward Highly Reversible Magnesium-Sulfur Batteries with Efficient and Practical Mg[B(hfip) 4 ] 2 Electrolyte Zhirong Zhao-Karger,* a Runyu Liu, b Wenxu Dai, b Zhenyou Li, a Thomas Diemant, c B. P. Vinayan a, Christian Bonatto Minella, a Xingwen Yu, d Arumugam Manthiram, d R. Jürgen Behm, a,c Mario Ruben, b,e Maximilian Fichtner a,b a Helmholtz Institute Ulm (HIU) Electrochemical Energy Storage, Helmholtzstr. 11, D Ulm, Germany b Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), P.O. Box 3640, D Karlsruhe, Germany c Institute of Surface Chemistry and Catalysis, Ulm University, Albert-Einstein-Allee 47, D Ulm, Germany d Materials Science & Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, United States e Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS, Université de Strasbourg, 23 rue du Loess, BP 43, F Strasbourg Cedex 2, France Chemical Preparation and Characterization All the samples were handled in an argon-filled glove box (MBraun) with a recirculation system and water, under a controlled argon atmosphere (H 2 O and O 2 < 0.1 ppm). The chemical operations were carried out either on the bench under Ar ( %) using standard Schlenk techniques (with vacuum < 0.1 Pa) or in glove box. The glassware was heated with heat gun under vacuum prior to use. Dimethoxy ethane (DME) was distilled over sodium under argon atmosphere and stored over 3Å molecular sieves in glove box. Hexafluoroisopropanol (CF 3 ) 2 CHOH (99%) was purchased from abcr GmbH and dried over 4 Å molecular sieves. NaBH 4 (98%) and anhydrous MgCl 2 (99%) were purchased from 1

2 Sigma Aldrich; Mg powder (625 mesh, 99%) was purchased from Alfa Aesar and used as received. 1 H, 13 C and 27 Al NMR spectra were recorded with a Bruker Advance II 500 spectrometer. The 13 C NMR spectra were broadband 1 H decoupled. A 1 M solution of Al(NO) 3 in D 2 O was used as reference for the 27 Al NMR. THF-d 8 or CDCl 3 were used as solvents for NMR measurements and the chemical shifts were reported in ppm using the residual solvent peak as reference. Simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectrometry (TGA-DSC-MS) were conducted with a Setaram thermal analyzer SENSYS evo TGA-DSC equipped with a Pfeiffer OmniStar mass spectrometer for the analysis of the evolved gas. Powder X-ray diffraction (XRD) patterns were recorded in the 2θ range using a Philips X pert diffractometer equipped with Cu Kα source. Scanning electron microscopy (SEM) in combination with Energy-Dispersive X-ray Spectroscopy (EDX) was performed in a Zeiss LEO 1530 with EDX detector X-maxN from Oxford instruments using carbon tape as the substrate. X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5800 MultiTechnique ESCA system (Physical Electronic). The spectra were acquired using monochromatic Al K α ( ev) radiation. The C 1s line with a binding energy of ev was used as reference. Synthesis of Mg(BH 4 ) 2 The mixture of MgCl 2 (3.81 g, 40 mmol) and NaBH 4 powder (3.09 g, 80 mmol) was ballmilled under argon at 200 rpm for 16 h. The finely ground mixture was transferred to a Schlenk flask equipped with a Dimroth condenser and 350 ml diethyl ether was added, then the reaction mixture was refluxed for 24 h. Subsequently, the white precipitate (NaCl) was filtrated under argon and the clear filtrate solution was condensed in vacuum. The resulting solid solvate [Mg(BH 4 ) 2 ] 6Et 2 O was dried in a vacuum at 0.1 Pa at gradually elevated 2

3 temperatures. Typically the crude product was at first heated at 40 C, and then the temperature was increased up to 100 C by 20 C per step until achieving a vacuum of 0.1 Pa for each step. Finally, 1.83 g (83%) of Mg(BH 4 ) 2 was obtained. 1 H-NMR ( MHz, d 8 - THF): δ = (q, J(B, H) = 80 Hz BH 4 ), 11 B NMR ( MHz, d 8 -THF, H-decoupled): δ = Synthesis of Mg[B(hfip) 4 ] 2 3DME Mg(BH 4 ) 2 powder (0.80 g, 14.8 mmol) was dissolved into 50 ml DME in a Schlenk flask. 8.2 equivalents of HOC(H)(CF 3 ) 2 (20.40 g, 12.8 ml, mmol) were slowly dropwise added into the stirred solution. After stirring at room temperature for 1h, the flask was equipped with a Dimroth condenser and the reaction was refluxed at 85 C under argon for 2h. After cooling down, the solvent was removed by vacuum. The resulting solid was further dried at gradually elevated temperatures from 30 to 60 C until achieving a vacuum of 0.1 Pa g (88%) of the final product was yielded. 1 H-NMR ( MHz, d 8 -THF): δ = 3.27 (s, DME- CH 3 ), 3.43 (s, DME-CH 2 ), 4.72 (m, CH). 19 F NMR ( MHz, d 8 -THF, H-decoupled): δ = (s, CF 3 ). 11 B NMR ( MHz, d 8 -THF, H-decoupled): δ = Preparation of the electrolyte solution The solid Mg[B(hfip) 4 ] 2 3DME was dissolved in a volumetric flask with proper amount of DME for the desired concentration. The molar concentration of the electrolyte is based on the molar mass of Mg[B(hfip) 4 ] 2 3DME. Preparation of magnesium polysulfide (MgS x ) solution The mixture of g (64.0 mmol) of sulfur powder and g (8.0 mmol) of Mg powder was ball-milled at 200 rpm for 10 h using silicon nitride vial and balls under Ar atmosphere. Subsequently, the powder material was transferred to a glass vial in glove box and 30 ml of 3

4 diglyme was added. The suspension was then stirred at 60 C for 3 days. The initially nearly clear upper liquid layer turned orangey red indicating the formation of MgS x. The filtrate MgS x solution is shown in Figure S1. Since there were still solid residues left after stirring, the concentration of MgS x in the solution can be roughly estimated to be < 7 wt% at this moment. Figure S1. Photograph of the mixture of Mg and sulfur powder in diglyme (1) and MgS x diglyme solution (2). Preparation of ACCS composites The ACC (Kynol Inc. ACC C) woven with a thickness of 0.4 mm was cut into discs with a diameter of 12 mm for the cathode preparation. The ACC discs were dried in vacuum in a rotating oven at 230 C for 48 h prior to use. The sulfur powder (99.98%, Aldrich) was spread on the ACC disc with a weight of 1.2 mg cm 2. Then, the ACC discs were heated in an autoclave at 160 C for 20 h. To remove the bulk sulfur on the surface of the fibers, the composite was further heated in a closed quartz tube at 300 C for 1 h and the composite denoted as ACCS. S-loading was determined by subtracting the mass of blank ACC from the ACCS composite and also characterized by TGA-DSC with a helium carrier gas flow at 20 ml min -1 and a heating rate of 10 C min -1. 4

5 Figure S2. XRD patterns ACC ACCS ACCS Carbon Sulfur Figure S3. SEM and EDX mapping images of ACC and ACCS. 5

6 Figure S4. TGA-DSC profiles for the composites prepared at 160 C and 300 C denoted as ACCS-160C and ACCS-300C, respectively. Figure S5. Mg stripping/plating of the Mg Mg cell with 0.3 M MgBhfip/DME electrolyte at a current density of 0.5 ma cm 2, interrupted at cycle 10, 50 and 100 for the EIS measurements. The electrodes at several electrochemical stages were taken out of the Swagelok cell, washed with DME, dried and then placed onto the SEM sample holder. 6

7 (a) (b) Figure S6. (a) SEM image of a Mg-foil as received. The table reports the average EDXvalues collected on several positions of the Mg-foil surface. (b) SEM image of a Mg-foil soaked for a week in Mg[B(hfip) 4 ] 2 electrolyte. The table reports the EDX-values collected on the position 6 and 7. Contaminations of carbon could be generated by a short exposition to air. 7

8 Figure S7. Charge-discharge profiles of the ACCS-Mg cell with 0.3 M MgBhfip/DME electrolyte at a C-rate of 0.1C. Figure S8. Charge/discharge profiles of the ACCS-Mg cell with a CNF-coated separator using 0.4 M electrolyte (85µl). 8

9 Figure S9. Cycling performance of the ACCS-Mg cells with 0.3 and 0.4 M MgBhfip/DME electrolytes and with the CNF-separator at a C-rate of 0.1C. Figure S10. CVs of ACCS with a three-electrode setup at a scan rate of 0.1 mv s 1. 9

10 Figure S11. SEM image of a Mg-anode collected from a symmetric Mg Mg cell in MgS x containing Mg[B(hfip) 4 ] 2 electrolyte. The anode presented dark spots over the surface. The table reports the EDX-values collected on the position 4, 5 and 6. Contaminations of carbon could be generated by the DME solvent used to wash the foil before the SEM analysis. Figure S12. SEM images of a Mg-anode obtained from a symmetric Mg Mg cell in Mg[B(hfip) 4 ] 2 electrolyte. The anode presented a bright surface. The holes are attributed to magnesium which has been stripped from the surface over cycling. The table reports the EDX-values collected on the position 1 and 2. Contaminations of Si, F, Al and Na and K are 10

11 generated by traces of both electrolyte and separator still present on the anode surface regardless the preliminary washing in DME before the SEM analysis. Figure S13. SEM images of the Mg-anodes obtained from the ACCS-Mg cells in Mg[B(hfip) 4 ] 2 electrolyte after cycling. After 18 cycles, sulfur can be detected on the Mg anode by EDX. Figure S14. Photograph of a Swagelok cell. 11

12 Figure S15. Photograph of a PAT-Cell from EL-Cell. More information can be found on the website: 12